In modern fail-safe circuits of the type used, for example, in supply circuits of machine tools, gates, furnaces and medical equipment, dual-channel switching on and off is required so that an inadvertent operation of only one channel will not result in the supply circuit being turned on. It is also required that when one channel fails, such as by contact welding, the other channel is still able to turn off.
An example of such a fail-safe circuit is found in DE 44 41 171 C1, This known circuit includes two relays with the coil of each relay being connected to a contact of the respective other relay in such a way that the relays will monitor each other, and ting on the supply circuit of the machine being controlled will take place only when both relays function properly. However, the presence of two relays renders the known circuit relatively complex.
DE 37 05 918 A1 discloses an electromagnetic relay having a magnetic system with a single coil penetrated by an iron piece of an overall U-shaped configuration. One leg of the iron piece is split in two parts so that two parallel magnetic circuits each having an associated clapper-type armature are provided on the same side of the coil. This arrangement is intended to ensure that if the contact driven by one armature undergoes contact welding, the entire magnetic flux will pass through this armature with the result that the other armature cannot be operated when the coil is energized a new. While this relay allows the switching of two circuits in a some what independent fashion, the separation between the, circuits is insufficient to satisfy the above-mentioned fail-safe requirements.
U.S. Pat. No. 4,833,435 describes an electromagnetic relay having a magnetic system with two separate U-shaped iron pieces extending in parallel through a common coil. Each iron piece is part of an individual magnetic circuit for operating an armature actuating a corresponding contact couple. The arrangement is intended to make sure that when one of the contact couples becomes welded, the other one can still open. This prior-art magnetic system suffers from high coil loss and from heat problems resulting therefrom.
AT 221 148 B discloses an electromagnetic relay with a coil surrounded by a shell-type two-piece yoke. Either yoke piece is formed of sheet iron by stamping and bending. Integrally formed with the yoke pieces are lugs which extend in parallel through the interior of the coil, Either yoke piece is provided with one or more clapper-type armatures which operate in synchronism upon energization of the coil. This type of relay is neither intended nor suited for the type of two-channel operation of fail-safe switching circuits referred to above.
It is an object of the invention to overcome at least part of the draw-backs existing with comparable prior-art magnetic systems for electromagnetic relays. It is a more specific object to provide a magnetic system for a relay which is suited for use in a fail-safe switching circuit at small coil losses.
To meet this object, the invention provides a magnetic system for an electromagnetic relay, comprising a coil arrangement defining a coil axis, and at least two magnetic circuits, each magnetic circuit including an iron piece and an armature, for operating an associated contact system, wherein the iron pieces are magnetically separated and extend parallel to the coil axis through the entire length of the coil arrangement, wherein the spacing between the iron pieces inside the coil arrangement is substantially smaller than the largest cross-sectional dimension of any one of the iron pieces.
In the present specification, the term xe2x80x9ciron piecexe2x80x9d is used to designate the overall structure of that component of the magnetic system which includes a portion (xe2x80x9ccorexe2x80x9d) extending inside and through the relay coil or coils, and portions (xe2x80x9cyokesxe2x80x9d) extending from the coil and cooperating with a relay armature.
Due to the close arrangement of the iron pieces inside the coil arrangement, a small coil cross-section, thus small coil losses, can be realized, essentially all of the magnetic flux produced by the coil arrangement is coupled into the magnetic circuit and available for actuating the armatures, and stray fluxes are largely avoided.
Surprisingly, it has turned out that inspite of the close arrangement of the iron pieces, the magnetic circuits are sufficiently uncoupled to obtain the kind of independent switching behavior of the contact systems operated by these circuits that is required for fail-safe circuits.
The small coil loss which results from the small cross-section of the coil arrangemnent and the fact the magnetic flux is used by more than one magnetic circuit, and the reduction of stray fluxes lead to the further advantage that heat problems are reduced.
In accordance with a preferred embodiment, the iron pieces are shaped and disposed relative to each other so as to minimize the ratio of their overall circumference to their total area. The overall cross-section encompassing the iron pieces and the spaces therebetween is preferably square or, ideally, circular, thereby optimising the efficiency in making maximum use of the magnetic flux produced by the coil arrangement.
In another embodiment, the magnetic circuits lie in planes which are defined by the coil axis and the respective one of the armatures and are equi-angularly distributed round the coil axis. This results in a spatially uniform distribution of the magnetic flux, thus in a further optimization concerning coil losses.
It is of advantage for the use of the magnetic system in many relay applications if each magnetic circuit contains a permanent magnet.
In another embodiment, each armature is substantially H-shaped and mounted for pivotal movement about a bearing axis extending perpendicular to the coil axis, and includes two armature plates constituting parallel legs of the H-shape, with a permanent magnet being disposed between these legs. Coupling the magnetic flux of the coil to the individual magnetic circuits is thus facilitated.
Preferably in this embodiment, two magnetic circuits are provided, the bearing axes of the armatures are coaxial, and their permanent magnets are oppositely magnetized. Forces generated on actuation of the magnetic system are thereby balanced.
In yet another embodiment, each magnetic circuit includes a permanent magnet extending substantially parallel to the coil axis between ends of a C-shaped iron piece, the permanent magnet having an intermediate pole and two end poles of a polarity opposite to that of the intermediate pole, and an armature mounted for pivotal movement at an intermediate location of the permanent magnet.
In another preferred arrangement, four magnetic circuits are provided which lie in two substantially perpendicular planes.
In accordance with a further embodiment of the present invention, two magnetic circuits are provided, and the coil arrangement includes two coils adapted to be independently energized, the armatures being so arranged that both of them are actuated only when both coils are energized. In case of energization of only one coil, at most one armature will respond. Faulty operation of a power circuit provided with the relay may be prevented by proper wiring of the relay contact assembly similar to conventional fail-safe circuits. While the magnetic circuits have approximately similar responsiveness, no switching operation takes place if only one coil is energized; i.e., inadvertent energization will have no effect. It is only by energising both coils that both armatures will be operated.
If the armatures including their associated contact assemblies are difeferent in responsiveness, the additional advantage of a defined attraction sequence of the two armatures is achieved. For instance, the armature exhibiting lower responsiveness may be provided for operating a contact assembly designed to carry load current. At the same time, failure can be detected from fact that the armature with the higher responsiveness operates. Different responsiveness may be realized by different magnetization or spring characteristics or by non-symmetrical coil windings or by combinations of these measures.
The coil winding process is simplified if the coils are adapted to generate identical magnetic fluxes. Different coils, on the other hand, would permit varying the excitation necessary to hold the relay in its operative condition.
In accordance with another embodiment, at least one of the coils is adapted to generate a magnetic flux sufficient to hold both armatures in their operative positions. In this case, the relay may be operated such that the holding current required for the armatures is reduced and, consequently, loss and heat generation may also be reduced.